This invention relates to a system and method of melting raw materials, such as reactive metals, e.g. titanium, zirconium, nickel, cobalt, and their alloys. The molten material can subsequently be used to form ingots or castings. The invention is presently considered especially useful for forming small cross-sectional ingots, and/or ingots or castings that will later be converted into powder, where homogeneity of each granule of powder is of particular concern.
Small cross-sectional bars and castings of these metals are used throughout the aerospace, automotive, energy, and medical industries. They can be machined or forged into any number of shapes. They may be used as the feedstock to be drawn into wire.
Such bars are typically made from larger ingots which are incrementally heated to high temperatures and then forged down into the desired size. The forging process can lead to considerable yield loss—a 60-70% yield of usable metal is typical. This is mainly due to deformation of the ends of the ingot after a number of forging steps. In addition, it can take months for an ingot to await its turn in the queue to be forged. Still further, due to the relatively small surface area to volume ratio of the large ingots and associated cooling rates, the grain size of the finished product may be larger than desired.
For all these reasons, it is desirable to cast the ingots nearer to their desired final cross-sectional size, a feat which has heretofore not been accomplished for small cross-sectional ingots.
It is also desirable to ensure that the ingots are as homogeneous as possible, for reasons that will be apparent to those of ordinary skill in the art.
Furthermore, parts made from powdered metals are increasingly common. The powder is usually formed by grinding, or by remelting and atomizing, an ingot or casting that has been cast from a molten material. The parts can then be produced by consolidating the powder either directly into a final shape, or into a preform that is then machined. In most uses, it is usually very important that each powder particle be of the same composition. This can only be achieved by ensuring that the metal ingot or casting from which the powder is formed is homogeneous, which can in turn only be achieved if the molten metal from which the ingot or casting is made is homogeneous.
The most common method of ensuring homogeneity in the molten metal is to stir the molten metal. Another method, which is mentioned in U.S. Pat. No. 6,006,821 to Haun et al., dated Dec. 28, 1999, and assigned to the Applicant herein, uses an induction coil. It should be noted that the induction coil disclosed therein is powered separately from the plasma arc torch using an additional power source. U.S. Pat. No. 6,006,821 to Haun et al. is hereby incorporated by reference.
Metals such as titanium, zirconium, nickel, cobalt, and their alloys can be contaminated by the oxide refractories used to make induction furnaces. Therefore, these metals are typically melted in segmented water-cooled copper vessels, with an associated induction coil and its separate power source. However, this melting technique is only about 25% efficient thermally.
Other methods of melting metals to thereby form ingots are known in the art.